FIG. 7 shows the general configuration example of a circuit that supplies a reference voltage to an analog signal processing circuit.
The circuit shown in FIG. 7 has resistors R5, R6 and capacitor C2. Resistors R5, R6 are connected in series between power supply voltage Vcc and reference potential G. Capacitor C2 is connected between the middle connection point of the series circuit and reference potential G. The voltage generated at capacitor C2 is obtained by dividing power supply voltage Vcc by the series circuit of resistors R5, R6. It is supplied as reference voltage Vref to analog signal processing circuit 90.
In the circuit shown in FIG. 7, since power supply voltage Vcc supplied to analog signal processing circuit 90 is divided to generate reference voltage Vref, an abrupt change in power supply voltage Vcc when the power is turned on will directly affect reference voltage Vref. If reference voltage Vref changes abruptly, noise containing many high-frequency components will be present in the output of the analog signal processing circuit. The noise will cause popping noise in the output when the power is turned on, especially in an audio signal processing circuit.
In order to restrain the noise generated when the power is turned on, in the circuit shown in FIG. 7, capacitor C2 is connected between the output terminal of reference voltage Vref and reference potential G. Since resistors R5, R6 and capacitor C2 constitute a low-pass filter, even if power supply voltage Vcc rises abruptly when the power is turned on, reference voltage Vref will rise slowly with a certain time constant. As a result, the high-frequency components causing popcorn noise can be suppressed.
The circuit shown in FIG. 7 suppresses noise by using a low-pass filter made up of resistors and a capacitor; however, the time for reference voltage Vref to rise to a prescribed level after the power is turned on is determined by the element values of the resistors and the capacitor. Also, the rising waveform is determined by the circuit configuration. In other words, the rising waveform of reference voltage Vref cannot be set as desired as far as the circuit configuration is concerned. Consequently, it is difficult to properly adjust the tradeoff between restraining the output noise when the power is turned on in the analog signal processing circuit and shortening the rise time of reference voltage Vref. For example, if the output noise is well restrained, the time delay from the time when the power is turned on to the beginning of signal processing will be relatively long.
Also, since the rise time is determined by the time constant of the resistors and capacitor, for an audio signal processing circuit, the capacitance of the capacitor must be very large in order to suppress the noise in the audible frequency band. As a result, the size of the element will be increased.